Most efficient way to remove multiple items from a IList<T>

Question

What is the most efficient way to remove multiple items from an IList<T> object. Suppose I have an IEnumerable<T> of all the items I want to remove, in the same order of occurrence that in the original list.

The only way I have in mind is:

IList<T> items;
IEnumerable<T> itemsToDelete;
...

foreach (var x in itemsToDelete)
{
    items.Remove(x);
}

But I guess it's not efficient, because it has to go over the list from the beggining every time the method Remove is called.

Answer 1

As the number of items to remove gets larger, you will probably find traversing the list and checking each item against a hashset of "items to remove" is more efficient. An extension method like this might help:

static void RemoveAll<T>(this IList<T> iList, IEnumerable<T> itemsToRemove)
{
    var set = new HashSet<T>(itemsToRemove);

    var list = iList as List<T>;
    if (list == null)
    {
        int i = 0;
        while (i < iList.Count)
        {
            if (set.Contains(iList[i])) iList.RemoveAt(i);
            else i++;
        }
    }
    else
    {
        list.RemoveAll(set.Contains);
    }
}

I benchmarked using this little program below. (Note that it uses an optimized path if IList<T> is actually a List<T>.)

On my machine (and using my test data), this extention method took 1.5 seconds to execute vs 17 seconds for the code in your question. However, I have not tested with different sizes of data. I'm sure for removing just a couple of items RemoveAll2 will be faster.

static class Program
{
    static void RemoveAll<T>(this IList<T> iList, IEnumerable<T> itemsToRemove)
    {
        var set = new HashSet<T>(itemsToRemove);

        var list = iList as List<T>;
        if (list == null)
        {
            int i = 0;
            while (i < iList.Count)
            {
                if (set.Contains(iList[i])) iList.RemoveAt(i);
                else i++;
            }
        }
        else
        {
            list.RemoveAll(set.Contains);
        }
    }

    static void RemoveAll2<T>(this IList<T> list, IEnumerable<T> itemsToRemove)
    {
        foreach (var item in itemsToRemove)
            list.Remove(item);
    }

    static void Main(string[] args)
    {
        var list = Enumerable.Range(0, 10000).ToList();
        var toRemove = new[] { 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 
                              43,  47,  53,  59,  61,  67,  71,  73,  79,  83,  89,  97, 101,
                             103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167,
                             173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239,
                             241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313,
                             317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397,
                             401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467,
                             479, 487, 491, 499, 503, 509, 521, 523, 541, 547, 557, 563, 569,
                             571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643,
                             647, 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733,
                             739, 743, 751, 757, 761, 769, 773, 787, 797, 809, 811, 821, 823,
                             827, 829, 839, 853, 857, 859, 863, 877, 881, 883, 887, 907, 911,
                             919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997};
        list.RemoveAll(toRemove); // JIT 
        //list.RemoveAll2(toRemove); // JIT 

        var sw = Stopwatch.StartNew();
        for (int i = 0; i < 10000; i++)
        {
            list.RemoveAll(toRemove);
            //list.RemoveAll2(toRemove);
        }
        sw.Stop();
        Console.WriteLine("Elapsed: {0} ms", sw.ElapsedMilliseconds);
        Console.ReadKey();
    }
}

---

UPDATE (for @KarmaEDV's & Mark Sowul's comments below): If you need to use a custom equality comparer, the extension method could have an overload that takes such a comparer:

public static void RemoveAll<T>(this IList<T> iList, IEnumerable<T> itemsToRemove, IEqualityComparer<T> comparer = null)
{
    var set = new HashSet<T>(itemsToRemove, comparer ?? EqualityComparer<T>.Default);

    if (iList is List<T>)
    {
        list.RemoveAll(set.Contains);
    }
    else
    {
        int i = iList.Count - 1;
        while (i > -1)
        {
            if (set.Contains(iList[i])) iList.RemoveAt(i);
            else i--;
        }
    }
}